Muscle-powered cardiac assist systems such as the one labeled 100a in FIG. 1A have been developed to aid patients with chronically and unacceptably low cardiac output, and who cannot have their cardiac output raised to acceptable levels by traditional treatments such as drug therapy. (See G. L. Anstadt & W. E. Britz, Jr., Continued Studies in Prolonged Circulatory Support by Direct Mechanical Ventricular Assistance, 14 Trans. Amer. Soc. Artif. Int. Organs 297 (1968)). In such a system 100a the ventricles of a heart 102a are surrounded by a pneumatic cardiac cup 104a to assist the heart 102a (and thus raise caridac output) during scheduled contractions. During the systolic (contraction) phase of the cardiac cycle the cardiac cup 104b, made of a flexible membrane, is inflated by a fluid to increase external pressure upon the ventricles, thus causing them to contract to a greater degree than otherwise. During the diastolic (relaxation) phase of the cardiac cycle the cardiac cup 104a is deflated to reduce the external pressure upon the ventricles, allowing them to expand to their at-rest position. In usual practice the cardiac pump is coupled to an external air (working fluid) source that drives a diaphragm (not shown) within the cup 104a.
As a variation on this theme, muscle tissue can be used to provide power to drive the working fluid. This is accomplished in FIG. 1A by coupling a cardiac pump to a balloon 114a that is wrapped by muscle tissue 116a. The muscle tissue 116a is stimulated to contract (via leads 132a) by an implantable pulse generator (IPG) 128a that senses the cardiac cycle, usually from heart electrocardiogram (ECG) activity via a cardiac sensing electrode 130a. This mode of assist is in co-pulsation with the heart pumping action.
Another cardiac assist device has been disclosed by Nielson and Chiu, in the book Biomechanical Cardiac Assist Cardiomyoplasty and Muscle-Powered Devices, edited by Ray C. J. Chiu, Futura Publishing Co., Inc., Mount Kisco, 1986 pp. 141-150, which book is hereby incorporated by reference.
The cardiac assist system 100b in FIG. 1B is similar to that of FIG. 1A, but has an aortic blood pump 126b (which is coupled to the aorta 134b) instead of the cardiac cup 104a. The aortic blood pump 126b provides hemodynamic and cardiac assist according to a principle of operation not unlike a prior art temporary intra-aortic balloon pump. As with the cardiac cup 104a, the aortic blood pump 126b must be coupled to an outside, fluid-pumping source, either external to the body or alternatively to a fully implantable system powered by a muscle pump that contracts in response to stimulation emanating from an IPG.
In operation of the system in FIG. 1B, the IPG 128b senses the onset of the diastolic phase via the sensor lead 130b, and stimulation bursts are supplied via the stimulator leads 132b to the powering muscle tissue 116b. The stimulation bursts cause the muscle tissue 116b to contract and thus cause the balloon 114b to expel working fluid, which then causes a diaphragm (not shown) in the aortic blood pump 126b to inflate. Somewhat before the onset of the systolic phase, the muscle tissue 116b is allowed to relax, causing the diaphragm in the aortic blood pump 126b to deflate, which causes a sudden increase in vascular space, and an easing of the load the heart 102b must pump against. This mode of assist is in counter-pulsation with the heart pumping action.
The systems 100a and 100b have some disadvantages related to the use of the balloon pumps 114a and 114b, which are, inter alia, wrapping the muscle tissue 116a and 116b around the balloon pumps 114a and 114b necessitates cutting the muscle collateral circulation, which may eventually lead to ischemia followed by the death of the tissue. Further, because the surface area of the powering muscle tissue is more than adequate to cover the surface area of the balloon pumps, excess muscle which overhangs the balloon pumps tends to generate unwanted tangential forces on the balloon pumps during muscle contraction, thereby wasting available energy of the powering muscle tissue. Also, such arrangements 100a and 100b are typically only well-suited for the latissimus dorsi muscle, and are not practical for other skeletal muscles. Additionally, the shape and size of the balloon pumps 114a and 114b usually result in undesirably large protrusions from the implanted area of the body.